Dual-tuned microstrip resonator volume coil

ABSTRACT

A dual-tuned volume coil for performing MR imaging according to one embodiment includes an inner cylinder having a first coil structure disposed on an inner surface thereof and a second coil structure disposed on an outer surface thereof. The first coil structure operates at a first resonance frequency and the second coil structure operates at a second resonance frequency. The volume coil includes an outer shield disposed about the inner cylinder, with the first and second coil structure being connected to the outer shield by means of a plurality of capacitors.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. patent applicationSer. No. 60/670,605, filed Apr. 11, 2005, which is hereby incorporatedby reference in its entirety. The present application is also related toand incorporates by reference in its entirety, International PatentApplication No. PCT/US2004/027532, entitled MICROSTRIP COIL DESIGN FORMRI APPARATUS, filed on Aug. 23, 2004 and published as WO 2005/020793 A2on Mar. 10, 2005.

BACKGROUND

Microstrip radio frequency (RF) transmission line coil technology formagnetic resonance (MR) imaging developed by Insight NeuroimagingSystems, LLC (the assignee of the present application) has beensuccessfully applied to a diverse set of magnet systems, ranging from 3T human to 11.7 T animal scanners. The coils have been developed forlinear and quadrature mode of operation and can function in combinationwith separate receiver coils.

In conventional MR imaging applications, the RF coils are tuned to thehydrogen (¹H) resonance frequency determined by the main magnet fieldstrength. Depending on the biomedical application, however, it may provedesirable to extend the RF coil capabilities beyond proton resonanceimaging. For instance, by tuning to other atom resonance frequenciessuch as phosphorous, fluoride, or carbon, the range of applications forthe RF coils can be significantly extended.

SUMMARY

A dual-tuned volume coil for performing MR imaging according to oneembodiment includes an inner cylinder having a first coil structuredisposed on an inner surface thereof and a second coil structuredisposed on an outer surface thereof. The first coil structure operatesat a first resonance frequency and the second coil structure operates ata second resonance frequency. The volume coil includes an outer shielddisposed about the inner cylinder, with the first and second coilstructure being connected to the outer shield by means of a plurality ofcapacitors.

A dual-tuned volume coil includes a single inner coil structure having aplurality of microstrips that operate at two different resonancefrequencies to provide two imaging modes and an outer shield that iselectrically coupled to the inner coil structure. The widths of themicrostrips vary as a function of their location about the coil.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The foregoing and other features of the present invention will be morereadily apparent from the following detailed description and drawings ofthe illustrative embodiments of the invention where like referencenumbers refer to similar elements and in which:

FIG. 1A is a cross-sectional view of a dual-tuned resonator volume coilaccording to one exemplary embodiment of the present invention;

FIG. 1B is a perspective view of the dual-tuned resonator volume coilaccording to FIG. 1A;

FIG. 2A shows a reflection coefficient (S₁₁) as a function of frequencyfor a dual-tuned resonator volume coil in accordance with a firstembodiment of the present invention and at a lower frequency ofoperation;

FIG. 2B shows the B₁ field behavior for the dual-tuned resonator volumecoil in accordance with the first embodiment, at the lower frequency ofoperation;

FIG. 3A shows a reflection coefficient (S₁₁) as a function of frequencyfor the dual-tuned resonator volume coil in accordance with the firstembodiment of the present invention and at a higher frequency ofoperation;

FIG. 3B shows the B₁ field behavior for the dual-tuned resonator volumecoil in accordance with the first embodiment, at the higher frequency ofoperation;

FIG. 4A shows a reflection coefficient (S₁₁) as a function of frequencyfor a dual-tuned resonator volume coil in accordance with a secondembodiment of the present invention and at a lower frequency ofoperation;

FIG. 4B shows the B₁ field behavior for the dual-tuned resonator volumecoil in accordance with the second embodiment, at the lower frequency ofoperation;

FIG. 5A shows a reflection coefficient (S₁₁) as a function of frequencyfor the dual-tuned resonator volume coil in accordance with the secondembodiment of the present invention and at a higher frequency ofoperation;

FIG. 5B shows the B₁ field behavior for the dual-tuned resonator volumecoil in accordance with the second embodiment, at the higher frequencyof operation;

FIG. 6A shows a reflection coefficient (S₁₁) as a function of frequencyfor a dual-tuned resonator volume coil in accordance with a thirdembodiment of the present invention and at a lower frequency ofoperation;

FIG. 6B shows the B₁ field behavior for the dual-tuned resonator volumecoil in accordance with the third embodiment, at the lower frequency ofoperation;

FIG. 7A shows a reflection coefficient (S₁₁) as a function of frequencyfor the dual-tuned resonator volume coil in accordance with the thirdembodiment of the present invention and at a higher frequency ofoperation;

FIG. 7B shows the B₁ field behavior for the dual-tuned resonator volumecoil in accordance with the third embodiment, at the higher frequency ofoperation;

FIG. 8A shows a reflection coefficient (S₁₁) as a function of frequencyfor a reference volume coil operating at a lower frequency of operation;

FIG. 8B shows the B₁ field behavior for the reference volume coil at thelower frequency of operation;

FIG. 9A shows a reflection coefficient (S₁₁) as a function of frequencyfor reference volume coil at a higher frequency of operation;

FIG. 9B shows the B₁ field behavior for the reference volume coil at thehigher frequency of operation;

FIG. 10A shows a reflection coefficient (S₁₁) as a function of frequencyfor a dual-tuned, single layer resonator volume coil in accordance witha fourth embodiment of the present invention and at a lower frequency ofoperation;

FIG. 10B shows the B₁ field behavior for the dual-tuned, single layerresonator volume coil in accordance with the fourth embodiment, at thelower frequency of operation;

FIG. 11A shows a reflection coefficient (S₁₁) as a function of frequencyfor a dual-tuned, single layer resonator volume coil n accordance withthe fourth embodiment of the present invention and at a higher frequencyof operation;

FIG. 11B shows the B₁ field behavior for the dual-tuned, single layerresonator volume coil in accordance with the fourth embodiment, at thehigher frequency of operation;

FIG. 12A shows a reflection coefficient (S₁₁) as a function of frequencyfor a reference volume coil operating at a lower frequency of operation;

FIG. 12B shows the B₁ field behavior for the reference volume coil atthe lower frequency of operation;

FIG. 13A shows a reflection coefficient (S₁₁) as a function of frequencyfor reference volume coil at a higher frequency of operation; and

FIG. 13B shows the B₁ field behavior for the reference volume coil atthe higher frequency of operation.

DETAILED DESCRIPTION

In an exemplary embodiment, the present invention provides a dual-tunedRF transmit/receive coil 100 for MR imaging that has the ability to tuneselectively to two separate resonance frequencies.

The present invention relates to resonator volume coils 110, 120 thatenable imaging at two different resonance frequencies. In a firstexemplary embodiment, two widely spaced resonance frequencies, such as170 MHz (¹H imaging at 4 Tesla) and 40 MHz (¹³C imaging at 4 Tesla) areachieved using two separate coil structures 110, 120 placed on the innerand outer surfaces, respectively, of an inner cylinder and connected toan outer cylindrical shield 130 via fixed and/or variable tuningcapacitors 140. FIGS. 1A and 1B show cross-sectional and perspectiveviews, respectively, of such an embodiment. The aforementionedcapacitors 140 can be seen in FIG. 1B.

Several variants of the exemplary two-layer coil 100 of FIGS. 1A and 1Bare possible in accordance with the present invention and are referredto herein as variants I, II and III. In a first variant, variant I, theinner coil 110 of the two-layer coil is the lower frequency coil. Invariant II, the inner coil 110 is the higher frequency coil and invariant III, the inner coil 110 is the lower frequency coil and extendsbeyond the outer coil 120.

Simulations were conducted to compare the field linearity performance ofthe exemplary, two-layer coil 100 of the present invention to that of aconventional, single-layer microstrip coil, referred to as the“reference coil” and shown schematically in cross-section in FIGS. 8Band 9B and again in FIGS. 12B and 13B.

An exemplary embodiment of coil 100 according to the present inventionhas the following parameters:

Shield Support Cylinder

-   -   OD=8.313 in (211 mm)    -   ID=8.188 in (208 mm)    -   Wall thickness=0.063 in (1.6 mm)    -   Material=acrylic        Coil Support Cylinder    -   OD=7 in (178 mm) (location of outer coil)    -   ID=6.5 in (165 mm) (location of inner coil)    -   Wall thickness=0.25 in (6.4 mm)    -   Material=acrylic        Coil length=5 in (127 mm)        Number of strips=8 per layer (2 layers)        Using “optimal” strip width for single-layer coils (a function        of ID/OD):

Relative strip width=66.7% of center-to-center spacing

Inner coil strip width=1.701 in (43 mm)

Outer coil strips width=1.833 in (47 mm)

Outer coil is rotated 22.5° with respect to the inner coil.

Operating frequencies for the coil 100 in accordance with one embodimentof the present invention:

f_(low)=40.8 MHz

f_(high)=170.2 MHz

Table I summarizes the results of the simulations for the three variantsof coil 100 and the reference coil implementations.

TABLE I Lower frequency Higher frequency B₁ Normalized B₁ Normalized(linear), B₁, Mode (linear), B₁, Mode Coil Type Q μT/{square root over(W)} μT/{square root over (Q · W)} separation Q μT/{square root over(W)} μT/{square root over (Q · W)} separation Variant I: 396 6.61 0.33214.5% 320 0.517 0.0289 1.3% Dual coil, inner coil is the low frequencycoil Variant II: 326 4.53 0.251 11.5% 328 2.08 0.115 4.5% Dual coil,inner coil is the high frequency coil Variant III: 432 6.35 0.305 13.2%464 0.842 0.0391 27.1%  Dual coil, inner coil is the low frequency coiland extends beyond outer coil Reference 433 7.30 0.351  9.5% 699 4.610.174 9.4% design (single coil)

Exemplary embodiments of the aforementioned variants will now bedescribed in greater detail.

An exemplary embodiment of Variant I, in which the lower frequency coilis on the inside 110, has the following capacitor values:

-   -   C_(low)=651 pF    -   C_(high)=65.1 pF

For the lower frequency (e.g., 40.8 MHz) operation of this embodiment:

-   -   Q=396    -   B1=6.61 uT/sqrt(W)    -   B1(Q-normalized)=0.332 uT/sqrt(Q*W)    -   Mode separation=14.5%

FIG. 2A shows the reflection coefficient (S₁₁) as a function offrequency for this embodiment, at the lower frequency of operation.

FIG. 2B shows the B₁ field behavior (black contour: ±1 dB boundary) forthe same embodiment, at the lower frequency of operation.

For the higher frequency (e.g., 170.2 MHz) of operation of thisembodiment:

-   -   Q=320    -   B1=0.517 uT/sqrt(W)    -   B1(Q-normalized)=˜0.0289 uT/sqrt(Q*W)    -   Mode separation=1.3%

FIG. 3A shows the reflection coefficient (S₁₁) as a function offrequency for this embodiment, at the higher frequency of operation

FIG. 3B shows the B₁ field behavior (black contour: ±1 dB boundary) forthe same embodiment, at the higher frequency of operation.

An exemplary embodiment of Variant II, in which the higher frequencycoil is on the inside 110, has the following capacitor values:

-   -   C_(low)=798.7 pF    -   C_(high)=52.2 pF

For the lower frequency (e.g., 40.8 MHz) of operation of thisembodiment:

-   -   Q=326    -   B1=4.53 uT/sqrt(W)    -   B1(Q-normalized)=0.251 uT/sqrt(Q*W)    -   Mode separation=11.5%

FIG. 4A shows the reflection coefficient (S₁₁) as a function offrequency for this embodiment, at the lower frequency of operation

FIG. 4B shows the B₁ field behavior (black contour: ±1 dB boundary) forthe same embodiment, at the lower frequency of operation.

For the higher frequency (e.g., 170.2 MHz) of operation of thisembodiment:

-   -   Q=328    -   B1=2.08 uT/sqrt(W)    -   B1(Q-normalized)=0.115 uT/sqrt(Q*W)    -   Mode separation=4.5%

FIG. 5A shows the reflection coefficient (S₁₁) as a function offrequency for this embodiment, at the higher frequency of operation

FIG. 5B shows the B₁ field behavior (black contour: ±1 dB boundary) forthe same embodiment, at the higher frequency of operation.

An exemplary embodiment of Variant III, in which the lower frequencycoil is on the inside 110 and the inner coil 110 extends beyond theouter coil 120, has the following capacitor values:

-   -   C_(low)=554 pF    -   C_(high)=67.2 pF

In this exemplary embodiment, the microstrips of the inner coil 110extend by 0.5″ beyond both ends of the outer coil 120 for a total lengthof 6″.

For the lower frequency (e.g., 40.8 MHz) of operation of thisembodiment:

-   -   Q=432    -   B1=6.35 uT/sqrt(W)    -   B1(Q-normalized)=0.305 uT/sqrt(Q*W)    -   Mode separation: 13.2%

FIG. 6A shows the reflection coefficient (S₁₁) as a function offrequency for this embodiment, at the lower frequency of operation

FIG. 6B shows the B₁ field behavior (black contour: ±1 dB boundary) forthe same embodiment, at the lower frequency of operation.

For the higher frequency (e.g., 170.2 MHz) of operation of thisembodiment:

-   -   Q=464    -   B1=0.842 uT/sqrt(W)    -   B1(Q-normalized)=0.0391 uT/sqrt(Q*W)    -   Mode separation=27.1%

FIG. 7A shows the reflection coefficient (S₁₁) as a function offrequency for this embodiment, at the higher frequency of operation.

FIG. 7B shows the B₁ field behavior (black contour: ±1 dB boundary) forthe same embodiment, at the higher frequency of operation.

For comparison, the performance of the reference coil will now bedescribed.

For the lower frequency (e.g., 40.8 MHz) of operation, the value of thecapacitors used is 598 pF, and the resultant parameters are:

-   -   Q=433    -   B1=7.30 uT/sqrt(W)    -   B1(Q-normalized)=0.351 uT/sqrt(Q*W)    -   Mode separation=9.5%

FIG. 8A shows the reflection coefficient (S₁₁) as a function offrequency for the reference coil operating at the lower frequency ofoperation, whereas FIG. 8B shows the B₁ field behavior.

For the higher frequency (e.g., 170.2 MHz) of operation, the value ofthe capacitors used is 33.3 pF, and the resultant parameters are:

-   -   Q=699    -   B1=4.61 uT/sqrt(W)    -   B1(Q-normalized)=0.174 uT/sqrt(Q*W)    -   Mode separation=9.4%

FIG. 9A shows the reflection coefficient (S₁₁) as a function offrequency for the reference coil operating at the higher frequency ofoperation, whereas FIG. 9B shows the B₁ field behavior.

In yet a further exemplary embodiment in accordance with the presentinvention, a single coil is provided having relatively closely spacedresonances frequencies, such as, for example 170 MHz (¹H at 4 T) and 160MHz (¹⁹F at 4 T). In this embodiment, the two imaging modes (typicallydegenerate, used in quadrature) are moved to two different frequencies.An advantage is that the modes are orthogonal, and thus decoupled, whilehomogeneity and efficiency are preserved. It is noted that this coilconfiguration can be used at linear-only modes of operation for bothfrequencies.

FIGS. 10B and 11B show cross-sections of such an embodiment. As can beseen, the widths of the coil strips vary as a function of theirlocation. In an exemplary embodiment, the strip widths vary sinusoidallyfrom their nominal width, with the width of the strips at the 0° and180° positions being −30% of the nominal width, and the width of thestrips at the 90° and 270° positions being +30% of the nominal width.

As in the above described embodiments, capacitors (not shown) arecoupled between each strip of the coil and an outer cylindrical shield.In an exemplary embodiment, the values of the capacitors varysinusoidally as a function of the position of their corresponding strip.In an exemplary embodiment, the capacitors of the strips at the 0° and180° positions are −17.6% of the nominal capacitance, and the capacitorsof the strips at the 90° and 270° positions are +17.6% of the nominalcapacitance.

An exemplary embodiment of the coil has the following parameters:

Shield Support Cylinder

-   -   OD=7.25 in (184 mm)    -   ID=7 in (178 mm)    -   Wall thickness=0.125 in (3.2 mm)    -   Material=acrylic        Coil Support Cylinder    -   OD=6 in (152 mm)    -   ID=5.75 in (146 mm)    -   Wall thickness=0.125 in (3.2 mm)    -   Material=acrylic        Coil length=5 in (127 mm)        Number of strips=12        Nominal strip width=1.018 in (25.8 mm or 67.6% of        center-to-center spacing)

The operating frequencies are as follows:

-   -   f_(low)=160.8 MHz    -   f_(high)=170.2 MHz

Table II summarizes the results of simulations for the aforementionedembodiment and the reference coil.

TABLE II Lower frequency Higher frequency Normalized B₁ ± 1 dBNormalized B₁ ± 1 dB B₁, B₁, diameter, B₁, B₁, diameter, Coil Type QμT/{square root over (W)} μT/{square root over (Q · W)} mm Q μT/{squareroot over (W)} μT/{square root over (Q · W)} mm Dual 679 5.52 0.212 108699 5.19 0.196 104 tuned, single coil Reference 693 5.48 0.208 111 6975.35 0.203 111 single- tuned linear

The performance of an exemplary embodiment of the dual-tuned, singlelayer coil of the present invention will now be described.

In this exemplary embodiment, the capacitor values and strip widths areas follows:

-   -   C(nominal)=26.4 pF    -   C variation=sinusoidal: −17.6% at 0° and 180°, +17.6% at 90° and        270°    -   Strip width=1.004 in (25.5 mm or 66.7% of center-to-center        spacing)    -   Strip width variation=sinusoidal: −30% at 0° and 180°, +30% at        90° and 270°

For the lower frequency (e.g., 160.8 MHz) of operation of thisembodiment:

-   -   Q=679    -   B1=5.52 uT/sqrt(W)    -   B1(Q-normalized)=0.212 uT/sqrt(Q*W)    -   B1±1 dB diameter=108 mm

FIG. 10A shows the reflection coefficient (S₁₁) as a function offrequency for this embodiment, at the lower frequency of operation

FIG. 10B shows the B₁ field behavior (black contour: ±1 dB boundary) forthe same embodiment, at the lower frequency of operation.

For the higher frequency (e.g., 170.2 MHz) of operation of thisembodiment:

-   -   Q=699    -   B1=5.19 uT/sqrt(W)    -   B1(Q-normalized)=0.196 uT/sqrt(Q*W)    -   B1±1 dB diameter=104 mm

FIG. 11A shows the reflection coefficient (S₁₁) as a function offrequency for this embodiment, at the higher frequency of operation

FIG. 11B shows the B₁ field behavior (black contour: ±1 dB boundary) forthe same embodiment, at the higher frequency of operation.

For comparison, the performance of the single-tuned reference coil willnow be described.

For the lower frequency (e.g., 160.8 MHz) of operation, the value of thecapacitors used is 30.0 pF, and the resultant parameters are:

-   -   Q=693    -   B1=5.48 uT/sqrt(W)    -   B1(Q-normalized)=0.208 uT/sqrt(Q*W)    -   B1±1 dB diameter=111 mm (linear)

FIG. 12A shows the reflection coefficient (S₁₁) as a function offrequency for the reference coil operating at the lower frequency ofoperation, whereas FIG. 12B shows the B₁ field behavior.

For the higher frequency (e.g., 170.2 MHz) of operation, the value ofthe capacitors used is 24.9 pF, and the resultant parameters are:

-   -   Q=697    -   B1=5.35 uT/sqrt(W)    -   B1(Q-normalized)=0.203 uT/sqrt(Q*W)    -   B1±1 dB diameter=111 mm (linear)

FIG. 13A shows the reflection coefficient (S₁₁) as a function offrequency for the reference coil operating at the higher frequency ofoperation, whereas FIG. 13B shows the B₁ field behavior.

Having described embodiments of the invention with reference to theaccompanying drawings, it is to be understood that the invention is notlimited to those precise embodiments, and that various changes andmodifications may be effected therein by one skilled in the art withoutdeparting from the scope or spirit of the invention as defined in theappended claims.

1. A dual-tuned volume coil for performing MR imaging comprising: an inner cylinder having a first coil structure disposed on an inner surface thereof and a second coil structure disposed on an outer surface thereof, wherein the first coil structure operates at a first resonance frequency and the second coil structure operates at a second resonance frequency; and an outer shield disposed about the inner cylinder, with the first and second coil structure being connected to the outer shield by means of a plurality of capacitors.
 2. The volume of claim 1, wherein the first coil structure includes a plurality of first microstrips coupled to the inner surface and the second coil structure includes a plurality of second microstrips.
 3. The volume coil of claim 1, wherein the first coil structure operates at a lower frequency than the second coil structure.
 4. The volume coil of claim 1, wherein the first coil structure operates at a high frequency than the second coil structure.
 5. The volume coil of claim 1, wherein the first coil structure operates at a lower frequency than the second coil structure and the first coil structure extends beyond the second coil structure.
 6. The volume coil of claim 5, wherein the first coil structure includes a plurality of first microstrips coupled to the inner surface and the second coil structure includes a plurality of second microstrips, the first microstrips extending beyond both ends of the second microstrips.
 7. The volume coil of claim 1, wherein at least one of the first and second resonance frequencies is an atom resonance frequency selected from the group consisting of phosphorous, fluoride and carbon.
 8. A dual-tuned volume coil comprising: a single inner coil structure having a first setof microstrips and a second set of microstrips, spaced from the first set of microstrips, the first and second sets of microstrips operating at two different resonance frequencies to provide two imaging modes; and an outer shield that is electrically coupled to the inner coil structure, wherein widths of the microstrips vary as a function of their location about the coil.
 9. The volume coil of claim 8, wherein the widths of the microstrips have a nominal width and the microstrips vary sinusoidally from the nominal width.
 10. The volume coil of claim 8, wherein the microstrips are electrically coupled to the outer shield by means of capacitors.
 11. A dual-tuned volume coil comprising: a single inner coil structure having a plurality of microstrips that operate at two different resonance frequencies to provide two imaging modes, the microstrips being divided into a first set of microstrips that operate at one resonance frequency and a second set of microstrips that operate at a different resonance frequency, the first set of microstrips being spaced radially outward from the second set of microstrips, the first and second sets of microstrips providing two imaging modes; and an outer shield that is electrically coupled to the inner coil structure.
 12. The volume coil of claim 11, wherein the first and second sets of microstrips are concentric with respect to one another.
 13. A dual-tuned volume coil comprising: a single inner coil structure having a plurality of microstrips that operate at two different resonance frequencies to provide two imaging modes, the microstrips being divided into a first set of microstrips that operate at one resonance frequency and a second set of microstrips that operate at a different resonance frequency for providing two imaging modes, the second set of microstrips being offset from the first set of microstrips such that at least portions of the second set of microstrips is axially aligned with spaces formed between the microstrips of the first set; and an outer shield that is electrically coupled to the inner coil structure.
 14. The volume coil of claim 13, wherein the first and second sets of microstrips are concentric to one another, with the first set of microstrips being spaced radially outward from the second set of microstrips. 